virtio-gpu api: comment feature flags

Add comments to the existing feature flags,
documenting which commands belong to them.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190410114227.25846-2-kraxel@redhat.com